This invention relates to encapsulated integrated circuit and microelectromechanical systems (MEMS) devices. More particularly, this invention relates to the prevention, reduction, elimination or purification of outgassing and trapped gases in such devices.
The ability to maintain a low pressure or vacuum for a prolonged period in a microelectronic package is increasingly being sought in such diverse areas as display technologies, micro-electro-mechanical systems (MEMS) and high density storage devices. For example, computers, displays, and personal digital assistants may all incorporate devices which utilize electrons to traverse a vacuum gap to excite a phosphor in the case of displays, or to modify a media to create bits in the case of storage devices, for example.
Microelectromechanical systems (MEMS) are very small moveable structures made on a substrate using lithographic processing techniques, such as those used to manufacture semiconductor devices. MEMS devices may be moveable actuators, sensors, valves, pistons, or switches, for example, with characteristic dimensions of a few microns to hundreds of microns. One example of a MEMS device is a microfabricated cantilevered beam, which may be used to detect the presence of a particular material, for example, a biological pathogen, or which may be used in a high-Q gyroscope. By coating the MEMS cantilever with a suitable reagent, the pathogen may bind with the reagent resulting in mass added to the cantilevered beam. The additional mass may be detected by measuring a shift in the characteristic vibration frequency of the cantilevered beam. However, because air is viscous, the cantilevered beam may be required to operate in a vacuum, so that the viscosity of ambient air does not broaden the resonance peak. Accordingly, MEMS devices such as cantilevered beams may also require vacuum packaging, in order to increase the signal to noise level of the detector to an acceptable level. Many other microdevices may require encapsulation, such as infrared (IR) emitters or detectors, which may need to evacuate the environment around the device to prevent absorption of the infrared radiation.
The packaging of the MEMS device may be accomplished by bonding a lid wafer with a device wafer. The MEMS devices, such as the cantilevered beams or infrared devices, are first fabricated on the device wafer. The lid wafer may then be prepared by etching trenches or device cavities in the lid wafer which will provide clearance for the MEMS device on the device wafer, if additional clearance between device and lid is needed. Before bonding, the lid wafer is aligned with the device wafer, so that the device cavity in the lid wafer is registered above the device on the device wafer, providing clearance for the height of the MEMS device and for its anticipated range of motion.
The lid wafer and device wafer assembly may then be loaded into a wafer bonding chamber, which is then evacuated. The lid wafer is then permanently bonded to the device wafer with a hermetic bond, so that the evacuated environment within the device cavity does not equilibrate with the outside environment by leakage over time. The bond may be made by heating a bonding element, such as an adhesive which is applied between the lid wafer and the device wafer. A glass frit is a commonly used adhesive, which may form a hermetic seal when heated to about 450 degrees centigrade.
One of the major problems with vacuum packaging of electronic devices, including MEMS is the continuous outgassing of hydrogen, water vapor, carbon monoxide, and other components found in ambient air, and from the internal components of the electronic or MEMS device. Typically, to minimize the effects of outgassing, one uses gas absorbing materials commonly referred to as getter materials, disposed within the vacuum cavity surrounding the microdevice. In the case of a MEMS device that does not require a cavity for clearance between device and lid, the getter can be deposited on the flat lid wafer. Generally a getter material is a metal alloy, for example, an alloy of zirconium (Zr), vanadium (V), and iron (Fe) that is sputter deposited on the surface of the lid wafer.
After deposition on the substrate, most gettering materials may form a passivation layer over their exposed surfaces, such as an oxide layer over a metal material. This layer must be removed for the getter material to be operative in its environment. This step of removing the passivation layer is generally called activation of the getter. This activation is generally performed by heating the getter material in the bonding chamber, prior to, after or during bonding of the device wafer to the lid wafer. If this bonding takes place at, for example, 450 degrees centigrade, the bonding temperature may be sufficient to drive the passivation layer off, or into, the deeper portions of the getter material, exposing the getter material again to the environment of the device within the device cavity, and allowing it to getter the impurity gases. Therefore, in such a system using a glass frit adhesive; the getter may be activated when the device is encapsulated with the lid wafer, and the components are heated within the evacuated bonding chamber.
Accordingly, activation of the getter material may be compatible with high temperature adhesives such as glass frits, but may not be compatible with lower temperature adhesives such as metal, solder or metal alloy bonding materials. Furthermore, if activation must take place in the same environment as the bonding occurs, the components of the MEMS device must be able to withstand the activation temperatures. Many MEMS device employ metal layers, which cannot withstand temperatures in excess of a few hundred degrees centigrade, if that.
Therefore, a need exists for a methodology which activates the getters at lower temperatures. A method and apparatus is described herein that provides a low temperature activation for getter materials, which is compatible with low temperature adhesives and low temperature structures used in the MEMS devices. It may also be the case that the method below provides additional performance benefits of a getter in a micromechanical device in that the passivation (contaminated) layer is removed rather than driven in to the bulk of the material. This may provide a more aggressive getter or a getter with greater gettering capacity.